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Epidemiology

It accounts for the minority (~5%) of monozygotic twin pregnancies and ~1-2% of all twin pregnancies. The incidence is ~1 in 10000 of all pregnancies 2.

Pathology

It results from a separation of a single ovum at ~ 8-13 days following formation (i.e. later than with an MCDA pregnancy). By this time a trophoblast has already formed, yielding a single placenta. These fetuses share a single chorionic sac, a single yolk sac and a single amniotic sac. The twins are identical (and of course of the same gender).

Associations

This type of pregnancy carries a relatively high incidence of congenital anomalies 4.

Radiographic features

Ultrasound

First trimester

shows a twin pregnancy with a single gestational sac and a single yolk sac (differentiating from a DCDAand MCDA pregnancy)